1. Field of the Invention
This invention relates to an ink ribbon cartridge for use in typewriters, printers and the like and in particular to such an ink ribbon cartridge detachably mountable in typewriters, printers and the like and having a supply port and a take-up port wherein an ink ribbon is intermittently advamced in tension from the supply port to the take-up port as the printing operation proceeds.
2. Description of the Prior Art
A typical prior art ink ribbon cartridge is illustrated in FIG. 1 and it generally includes a bottom case 1 having a bottom wall and a side wall integrally formed along the periphery of the bottom wall and a top wall 2 which is placed on top of the side wall of the bottom case 1 to define a space therebetween in which ink ribbon 3 is stored. As shown, the cartridge is provided with a supply port 4 and a take-up port 5 spaced apart over a predetermined distance from the supply port 4 thereby defining a printing section therebetween. Thus, when printing operation is in progress, the ink ribbon 3 is fed outside to the printing section thrOugh the supply port 4 and then led into the cartridge again through the take-up port 5 to be stored therein by means of a driving mechanism (not shown). As is well-known in the art, when the cartridge is mounted in a printing machine, a printing head provided with a printing hammer and a type carrier carrying thereon a plurality of types is located in the space defined between the supply and take-up ports 4, 5 of the cartridge, so that a selected type is impacted on the ink ribbon against paper placed around a platen roller of the printing machine by the printing hammer to form a desired imprint on the paper. Thus, as the printing operation proceeds, the ink ribbon is advanced from the supply port 4 to the take-up port 5 through the printing section defined therebetween intermittently as well as incrementally.
In order to allow to form proper imprints by having the printing hammer impact on the lead-out portion of the ink ribbon 3, typically the so-called "back tension" is desired to be imparted to the ink ribbon 3 so that the ink ribbon 3 may be intermittently advanced from the supply port 4 to the take-up port 5 in tension. Accordingly, in a typical prior art ink ribbon catridge, a deterrent mechanism is provided to apply back tension to the ink ribbon 3. That is, as shown in FIG. 1, such a prior art deterrent mechanism is comprised of a ribbon guide post 7 formed at one end of a ribbon guide rib 6 which is integrally provided with the bottom case 1 as extending toward the supply port 4, and a leaf spring 8 whose base end is inserted into a recess formed in the bottom case and whose forward end is pressed against the ribbon guide post 7 with the ink ribbon 3 sandwiched therebetween. As a result, a deterrent force is imparted to the ink ribbon 3 as mainly determined by the pressing force applied by the leaf spring 8, so that the ribbon 3 may be kept in tension as it is pulled toward the take-up port 5 through the supply port 4.
However, in such a prior art deterrent mechanism, the ribbon guide post 7 projecting upwardly from the bottom wall of the case 1 into the space defined between the bottom and top walls must be so shaped that it is slightly convergent from its base to its top because of manufacturing constraints, and yet a gap must be provided between the top wall 2 and the top of the post 7 by taking into account reasonable manufacturing tolerances. Such being the case, it has been observed more often than not that the ribbon 3 at least partly gets into the gap defined between the top case 2 and the top of the guide post 7 as shown in FIG. 2a. Alternatively, the ribbon 3 has also been found to move into the gap defined between the top case 2 and the leaf spring 8, as shown in FIG. 2b, or into the gap defined between the bottom case 1 and the leaf spring 8, as shown in FIG. 2c. This is disadvantageous because the level of "back tension" fluctuates and the ink ribbon 3 cannot be maintained properly in tension thereby producing printed characters of poor quality.